There isn’t a single fixed shutter speed, because it depends on cloud speed, distance, and focal length. But for a rough example, at 80mm with clouds about 2000m away moving around 150km/h, about 1/125s can already be enough to freeze their motion.

So if your images looked soft at 1/125–1/160s, cloud motion may not have been the real problem. More likely causes are focus accuracy or the nature of the clouds themselves: soft, fluffy clouds often don’t have strong edges, which can make autofocus less precise and also make them appear less “sharp” even when motion blur isn’t significant.

If you want a safe approach, using a faster shutter speed like 1/500s won’t hurt when light allows, but don’t assume slower speeds automatically cause blur. Check focus carefully, and look for distinct cloud edges or contrasty areas if you’re relying on autofocus.